Holy barnacles! I don't even remember how I came across this, but yesterday I found this 1990 undersea monster flick starring R. Lee Ermey and Ray Wise directed by J.P Simon of Pieces and Slugs fame.
I see this this was also known as Endless Descent, for which I do remember the coverbox. I never realized there was a third straggler after Leviathan and Deep Star Six the year before. Consider me sold!